The present invention relates to protective devices that interrupt an electric current by fusing a low-melting metal element in the event of failure.
Protective devices comprising a heating element and a low-melting metal element stacked on a substrate have previously been known as protective devices that can be used to prevent not only overcurrent but also overvoltage (e.g., see Japanese Patent No. 2790433, JPA HEI 08-161990).
In the protective devices described in these patent documents, a current passes through the heating element in the event of failure so that the heating element generates heat to melt the low-melting metal element. The molten low-melting metal element is attracted onto the electrode on which the low-melting metal element is mounted on the electrode surface due to the good wettability, whereby the low-melting metal element is broken and the current is interrupted.
An alternative embodiment of connection between the low-melting metal element and the heating element in this type of protective device is also known from e.g. JPA HEI 10-116549 and JPA HEI 10-116550, according to which the low-melting metal element and the heating element are two-dimensionally arranged and connected to each other on the substrate rather than stacking the low-melting metal element on the heating element with the same result that the current supply to the heating element is interrupted upon fusion of the low melting metal element.
To meet the tendency toward size reduction of portable equipment, a means to reduce the thickness of this kind of protective device was proposed by providing a fuse (low-melting metal element) on a base substrate and sealing it with an insulating cover plate and a resin to reduce the thickness (e.g., see JPA HEI 11-111138).
Substrate-type temperature fuses according to this conventional technique comprise film electrodes formed on one side of a base substrate, a low-melting alloy piece bridged between the film electrodes, and a flux applied to the low-melting alloy piece. An outer insulating cover plate smaller than the base substrate is provided on one side of the base substrate, wherein a sealing resin is filled in a gap between the peripheral end of the insulating cover plate and the peripheral end of the base substrate, and the outer surface of the sealing resin between the peripheral end of the insulating cover plate and the peripheral end of the base substrate is a concavely curved sloped surface or a linearly sloped surface.